1. Field of the Invention
The present invention generally relates to surface acoustic wave devices and filters equipped with the same such as a transmit filter, a receive filter and a duplexer.
2. Description of the Related Art
The SAW device is widely applied to the band-pass filter and duplexer of the cellular phone. In recent years, the filter using the SAW device is required to have higher performance as the cellular phone has higher performance. One of the requirements for the SAW device is to improve the temperature stability because temperature change moves the pass-band frequency range of the filter. As is known, lithium tantalate (LiTaO3, hereinafter simply referred to as LT) is a piezoelectric material having a large electromechanical coupling coefficient, which is advantageous for realizing broad filter characteristics. However, LT has a disadvantage in that it is inferior to quartz crystal in terms of temperature stability. The piezoelectric material has a general tendency of incompatible characteristics such that materials having large electromechanical coupling coefficients such as LT and lithium niobate (LiNbO3, hereinafter simply referred to as LN) have comparatively poor temperature stability, while materials having good temperature stability such as quartz crystal have comparatively small electromechanical coupling coefficients. Thus, a piezoelectric material having both a large electromechanical coupling coefficient and a good temperature stability has been sought for years.
There are various proposals to realize materials having a large electromechanical coupling coefficient and a good temperature stability. For example, Yamanouchi et al. propose a substrate having an LN or LT substrate and an SiO2 film, which is grown thereon and has a temperature coefficient opposite to that of LN or LT (see IEEE Trans. On Sonics and Ultrasonics., vol. SU-31, pp. 51-57, 1984). Nakamura et al. achieve improvements in temperature stability by forming a polarization inverted region in the LT substrate surface that has a depth less than the involved wavelength and by utilizing the electrostatic short-circuit effect thereof (see Japanese Patent No. 2516817). Onishi et al. propose to improve the temperature stability by directly joining a thin piezoelectric substrate and another substrate made of a thick, low-expansion material and thus restraining contraction and expansion due to change of the temperature of the piezoelectric substrate (see Japanese Laid-Open Patent Application No. 11-55070, or H. Sato et al., Proc. Of IEEE Ultrasonics Symposium, pp. 335-338, 1998). Yamanouchi et al. propose to use adhesive for joining the two substrates (see, Yamanouchi et al., “Proc. of IEEE Ultrasonics Symposium”, pp. 239-242, November, 1999). Japanese Laid-Open Patent Application No. 9-208399 proposes to improve the filter performance by joining two different kinds of substrates in solid-phase reaction.
However, the above-mentioned proposals have problems to be solved. The proposal by Yamanouchi et al. has difficulty in controlling the thickness and quality of the SiO2 film, this resulting in frequency deviations of the SAW devices. Further, the proposal requires providing the silicon oxide film on the comb-type electrodes (interdigital transducers: IDT), this increasing the SAW propagation loss. It follows that the proposal may be applied to only limited SAW devices.
The proposal described in Japanese Patent No. 2516817 has a problem that the polarization inverted layer affects the performance of the SAW device and very fine depth control of the polarization inverted layer is required.
There are problems in the proposals described in Japanese Laid-Open Patent Application No. 11-55070 and taught by Onishi et al. The way of joining the low-expansion substrate to the piezoelectric substrate requires high-temperature annealing that follows the step of washing the substrates. It is therefore required to use, as the low-expansion substrate, a substrate having a small Young's modulus such as glass in order to prevent the wafer from being broken during the annealing process. Thus, distortion due to the difference in thermal expansion coefficient between the substrates thus joined is not transferred to the main substrate sufficiently, so that improvement in the temperature stability cannot be exhibited well.
The method of joining the piezoelectric substrate to the low-expansion substrate at room temperature by means of UV cure adhesive reduces stress by the adhesive at the joint interface. However, this reduces the degree of improvement in temperature stability.
The way of joining the substrates in the solid-phase reaction described in Japanese Laid-Open Patent Application No. 9-208399 requires the thermal treatment at 1000° C. or higher at which melting does not take place after the step of joining. This may change some constants of the piezoelectric substrate and thus causes velocity change, so that frequency deviations occur.